I recently updated from v25c to v26a for the following mods
DragonAPI, ChromatiCraft, ElectriCraft, ReactorCraft, RotaryCraft
Those are the only five mods that have changed and whenever my server tries to start up the world i get this and it closes.
Time: 28.05.20 00:45Time: 28.05.20 00:45Description: Exception ticking world java.lang.NoSuchMethodError: net.minecraft.world.IBlockAccess.func_72807_a(II)Lnet/minecraft/world/biome/BiomeGenBase; at Reika.ChromatiCraft.ChromaticEventManager.stopBiggerOakDecay(ChromaticEventManager.java:297)
Full Log: https://pastebin.com/MPMz1QnX
When i go in and copy the whole world folder to my client and load it up just like your regular save it works flawless.